Copending application Ser. No. 214,250 filed July 1, 1988 by the applicants of the instant invention (incorporated herein by this reference) describes that one type of phase modulation data transmission systems is a differential detection system which detects transmitted data by looking at changes in the phase of the input differential phase modulated carrier signal.
Briefly, two basic methods (or systems) are known for demodulating a differential phase modulated carrier signal: a coherent method and a non-coherent method.
In the coherent method, a carrier recovery circuit is used to reconstruct in-phase and quadrature-phase reference signals which are multiplied against the received signal and a phase shifted version of the received signal. The results are then linearly combined to produce a pair of demodulated signals representing the two modulation components of the received signal. In the non-coherent demodulation method, no carrier recovery circuit is used. Instead, a delayed version of the received signal is multiplied against the received signal to produce the demodulated signal.
Non-coherent demodulation has the advantage of being simple to implement, as it does not require a carrier recovery circuit. Moreover, the input filtering is less complex since a phase splitting filter is not required to generate the complex form of the received signal. However, for the non-coherent method, typically a post detection filter is needed to eliminate double frequency terms generated by the multiplication process.
In digital signal processing (where demodulating functions are performed using digital number values obtained from a periodic sampling of the received signal) the coherent method becomes more attractive because many of the calculations can be done at the symbol or baud rate which may range from approximately 600 Hz to 2400 Hz. Digital implementation of the non-coherent method requires operations at the higher sampling rate of at least 8 KHz, for the post detection filter.
The copending '250 application discloses that estimates of the in-phase and quadrature-phase components of the input demodulated carrier signal may be used in the demodulation process for generating a socalled "pseudo-coherent" demodulator. To do this, the '250 device has the structure of a typical coherent demodulator, but does not have any carrier recovery circuits, using instead the instantaneous estimates of the input carrier signal components. These carrier estimates are obtained directly from the received carrier signal, with a delay of one baud between the calculation of the carrier estimates and their use in the demodulator, for effecting a differential phase shifted demodulated signal at the output of the demodulator.
Although this demodulator is "pseudo-coherent", the fact remains that it still is not coherent per se. Therefore, in order to obtain better signal-to-noise ratios and better modem performance, a coherent demodulator is still deemed desirable.